In a common-rail fuel injection system, the functions of pressure generation and fuel injection are separate by means of an accumulator volume (precisely, the common fuel rail) and the injection high pressure is generated and maintained independent of the engine speed and the injected fuel quantity.
Fuel under pressure is supplied to the common rail by means of a continuously operating high-pressure pump driven by the engine. The required pressure inside the rail, which is variable dependent on the engine operating point, is controlled by a metering unit of the fuel mass located at the inlet of the high-pressure pump at the low-pressure side of the injection system. The metering unit makes sure that the pump delivers the right quantity of fuel to the rail according to the current fuel pressure measured by a rail-pressure sensor in a closed loop control driven by an electronic control unit. The pressure control performance are however poorer in comparison with a control based on a valve which can act straight on the rail on the high-pressure side. Actually, a slower fuel pressure response can lead to increasing emissions and noise, as well as to undesired overpressure in case of fault of one injector.
To the purpose of overcoming the above disadvantages, a pressure-relief valve connected to a return line to a fuel tank is also traditionally fitted to the fuel rail (thus, on the high-pressure side of the fuel injection system) in order to act as a pressure control valve, particularly for draining fuel not required for injection in a control loop rapidly reacting to changes in the engine operating point (e.g., in the event of negative load changes, such as the release of the accelerator pedal by the driver), as well as to limit fuel pressure in the fuel rail to the maximum permissible pressure, in which case it acts as a pressure limiter.
In case of a fault on the metering unit (i.e., an electrical or mechanical fault that causes the valve of the unit to be blocked open) the rail pressure can reach dangerous levels for the integrity of the components of the engine and even of the vehicle, and the pressure-relief valve acts to drain the fuel in excess in the common rail to a re-circulating line conveying it back to the tank.
However, such a pressure-relief valve is an expensive component and it would be desirable to achieve a significant cost reduction in common-rail systems.
EP 0 896 144 discloses a fuel injection control apparatus for an accumulator type engine, wherein an electronic control unit is adapted to actuate the injectors during an ineffective injection control time so that they release fuel to the fuel tank through a return pipe in order to reduce the fuel pressure in the common rail.
GB 2 332 241 discloses an accumulator fuel injection systems for vehicle diesel engines wherein an electronic control unit is adapted to turn on a solenoid valve of an injector to cause the pressurized fuel supplied from the common rail to a controlled chamber of the injector to be drained to the fuel tank on a lower pressure side.
At least one aim of the present invention is to improve the engine performances without increasing the costs, and specifically to develop a strategy for effectively controlling the operating high pressure in a cost-effective common-rail fuel injection system lacking any pressure relief valve on the high-pressure side. More specifically, it is at least one object of the present invention to develop a cost-effective strategy for controlling the high pressure in a fuel injection system in order either to track a variable pressure set-point determined as a function of the engine operating point or to prevent serious damages to the system components due to the pressure raising above an admissible range of pressure set-points (e.g., because of a fault in the pressure regulating means provided on the low pressure side of the system). In addition, other aims, objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.